1. Field of the Invention
The invention relates to a vehicle steering system.
2. Discussion of Background
Japanese Patent Application Publication No. 10-278826 (JP 10-278826 A) describes a so-called steer-by-wire steering system in which a steering member and steered wheels are not mechanically coupled to each other. JP 10-278826 A suggests that there are provided a main steering angle sensor and a backup steering angle sensor that is used in place of the main steering angle sensor in the event of a failure of the main steering angle sensor. In addition, Japanese Patent Application Publication No. 2004-90784 (JP 2004-90784 A) describes a steer-by-wire steering system in which a planetary gear mechanism is arranged between a steering member and a steered mechanism. JP 2004-90784 A suggests that, in the event of a failure of a steering angle sensor, rotation of a ring gear of the planetary gear mechanism is restrained to allow manual steering via the planetary gear mechanism of which the gear ratio has been fixed through the restraint of the rotation of the ring gear.
According to JP 10-278826 A, the multiple expensive steering angle sensors are used, which increases the manufacturing cost. According to JP 2004-90784 A, the planetary gear mechanism is used, which also increases the manufacturing cost.